The invention relates to carbon fiber sheet materials and methods of making and using the same. More particularly, the invention relates to carbon fiber structures formed of substantially parallel carbon fibers and having an adhesive polymeric material adhered directly onto one or more surfaces thereof, which structures are useful as electrodes in electrical energy storage devices.
Carbonaceous materials, such as those derived from pitch or polyacrylonitrile (PAN), can be used as an electrode material in electrical energy storage devices. For example, carbon can be used as a component of the electrode in primary batteries, primary fuel cells, secondary fuel cells, secondary batteries and capacitors. The carbon material functions as a current collector and/or as a reactive material to form new compounds which have different structures and properties than the original carbon material, and most recently, as semiconductor materials which form salts with ions of the electrolyte. Examples of carbon fiber based electrodes and batteries incorporating the same as a component are described, for example, in U.S. Pat. Nos. 4,865,931; 5,518,836; 4,830,938; 4,952,466; and 4,929,521.
In general, the batteries include electrodes formed of electrically conductive carbon fibers. The carbon fiber electrodes are placed in a suitable housing in contact with an electrolyte, typically an ionizable salt in a nonaqueous fluid. The electrodes are separated from one another in the housing to prevent short circuiting between the electrodes while allowing ions to travel between the electrodes. Typically the electrodes are separated from one another by materials that are separate and discrete from the individual electrodes. Examples of separators described in the art include sheets of fiberglass, nonwoven polymeric sheets or webs, coated metallic screens, porous films, and spacers (such as polymeric beads) extending between the electrodes.
Such batteries can offer an attractive alternative to traditional lead-acid reduction-oxidation batteries. For example, batteries including a carbon fiber based electrode can generate high power, and are generally efficient, compact, and non-toxic. However, there are difficulties associated with the production of carbon fiber based electrodes. It can be difficult to handle the carbon fiber assemblies, particularly on a commercial production scale. Carbon fiber assemblies, particularly those prepared from carbon fiber tows, have limited dimensional stability and are difficult to handle without touching and compromising the carbon fiber surface. This in turn can slow fabrication times in manufacturing the batteries and limit the reasonably expected quality and performance of batteries and components. Yet traditional techniques for stabilizing fibrous assemblies in the textile art are not readily translated into the production of carbon fiber assemblies for battery applications.
For example, carbon fiber tows have been impregnated with resin to form prepreg tapes and rovings. However, the impregnating resin would be expected to interfere with the electrical performance of the electrode. Also, impregnating a carbon fiber tow with resin would reduce the available surface area of the carbon fibers, which can also comprise performance in battery applications.
Woven and/or knit carbon fiber assemblies are typically more dimensionally stable than a fiber tow. However, it can be difficult to weave or knit carbon fibers without a size on the fiber surfaces because of the inherent stiffness and brittle nature of these fibers. The size, however, can be difficult to remove from the woven and/or knit assembly. If left on the fibers, the sizing can interfere with the electrical performance of the carbon fiber electrode. Still further, woven and knit carbon fiber assemblies can have significantly reduced percent surface area available for interaction with the electrolyte solution. Also, the surface geometry of a knit or woven fabric is irregular, which can cause electrical inefficiencies because ions will transfer preferentially to the high peaks of the fabric.
The present invention provides carbon fiber sheet materials, which are useful for a variety of applications, including use as electrodes in electrical energy storage devices. The carbon fiber sheets of the invention include a network formed of a plurality of carbon fibers or filaments, arranged substantially parallel relative to one another. To provide improved dimensional stability and ease of handling, the carbon fiber networks include an adhesive polymeric material adhered directly onto one, and preferably both, surfaces of the carbon fiber network as an integral part of the carbon fiber network. The adhesive polymeric material is in the form of a porous or permeable layer so as to allow the passage of ions therethrough and into contact with the carbon fiber network. Advantageously the adhesive layer can be releasably adhered to the surface of the carbon fiber network.
In one advantageous embodiment of the invention, the adhesive polymeric material is a melt blown web which is melt blown and adhered directly onto one or more surfaces of the carbon fiber network. However, the adhesive material can also be present in other forms, such as but not limited to, other types of fibrous webs (such as spunbonded webs), microporous films, a discontinuous pattern of adhesive, and the like.
Despite the presence of the adhesive on a surface of the carbon fibers, the electrical performance of the carbon fibers as an electrode is not significantly compromised. In this regard, the adhesive material is applied so that only a small percentage of the carbon fiber surface is used or contacted by the adhesive. This in turn allows ions to be less restricted in their movement.
The resultant stabilized carbon fiber assemblies can be more readily handled, particularly when fabricating batteries, thus reducing production times and costs. Further, the carbon fiber networks of the invention can provide cost benefits by providing an integral separator/carbon fiber electrode assembly, in contrast to conventional separators, which are discrete and separate from the electrode.
The carbon fiber network can be a woven, knit or nonwoven substrate. In one advantageous embodiment of the invention, the carbon fiber network is formed from one or more tows or bundles of carbon fibers. Preferably the fibers of the tow(s) are spread out relative to one another to form a substantially planar sheet prior to applying the adhesive layer web to a surface thereof. The added flexibility in production provided by stabilizing the tow with the adhesive layer is particularly advantageous because spreading the fibers of the tow to form a planar sheet increases the surface area available for interaction with the electrolyte solution.
This also has the advantage of improving control of the uniformity of the tow thickness across the width and length dimensions thereof. As a result, carbon fiber sheet materials can be manufactured that have a substantially uniform thickness. This in turn can be particularly advantageous for battery performance. In this regard, battery performance is related at least in part to providing a substantially uniform or homogeneous ratio of electrolyte mass to carbon fiber mass. Thus battery performance can be improved by minimizing inconsistencies in the distances between fibers, and thus the distances required for ions to travel between carbon fibers.
In another embodiment of the invention, the carbon fiber structure is a woven or nonwoven web formed of intersecting transverse (weft) yarns and longitudinal (warp) yarns. The warp yarns are formed of carbon fibers or filaments and can be mono-filament or multifilament yarns. Preferably the weft yarns are thermoplastic, polymer coated fiberglass yarns, and more preferably polyolefin coated fiberglass yarns, such as polypropylene coated fiberglass yarns. Alternatively, the weft yarns can be formed of a thermoplastic material, such as an amorphous (or atactic) polyolefin. The thermoplastic polymer coated yarns and/or thermoplastic polymer yarns can also be multifilament or mono-filament yarns. The thermoplastic polymer coated weft yarns and/or thermoplastic polymer weft yarns can further improve the dimensional stability of the assembly by providing additional bonding with the adjacent adhesive layer.
Preferably the adhesive layer is formed of an adhesive material that is capable of being fiberized, i.e., being formed into a fibrous structure. The adhesives can be thermoplastic or thermoset adhesives. Particularly preferred adhesives include polyolefins, and more preferably amorphous (i.e., atactic) polypropylene polymers. The basis weight of the adhesive layer can vary, and generally ranges from about 1 to about 100 grams per square meter (gsm), depending upon a variety of factors, such as the specifics of the carbon fiber network structure, end use of the product, and the like. Basis weights outside of this range can also be used. In one advantageous embodiment, the adhesive layer is a melt blown web having a relatively small basis weight, from about 1 to about 35 gsm, although webs having a basis weight outside this range can also be used. Thus low basis weight materials can be successfully integrated with the carbon fibers to form a unitary carbon fiber network to provide the benefit of insulating the carbon fiber layers without high material costs. The porous or permeable adhesive layer also can stabilize the carbon fiber network and improve ease of handling.
In one particularly advantageous embodiment of the invention, the carbon fiber assemblies have porous or permeable adhesive layers, preferably in the form of melt blown webs, on opposing surfaces of the carbon fiber sheet. In this embodiment of the invention, the adhesive layers can extend beyond at least one, preferably two, and more preferably three, of the peripheral edges of the carbon fiber sheet. A portion of the adhesive layers extending beyond the edge of the carbon fibers can be treated under conditions sufficient, for example by application of heat and optionally pressure, to form a bond or selvage edge to the assembly. This in turn can provide a xe2x80x9cbagxe2x80x9d encapsulating the carbon fibers, thus further improving dimensional stability and ease of handling. For carbon fiber tows, this has the additional advantage of allowing the carbon fibers to spread out more readily without compromising handling or stability and increasing carbon fiber surface area available for interaction with the electrolyte solution.
A selvage edge can be prepared using other techniques as well. For example, the selvage edge can be prepared mechanically (for example, by applying pressure without significant heat), chemically (for example, using a plasticizer to soften the material), and the like. Preferably at least one edge of the carbon fiber network remains exposed, or is readily exposed, so as to allow ready attachment of electrodes.
In addition, the adhesive layers, particularly melt blown web(s), can be readily applied under conditions to control thickness, porosity, and the like. For example, the adhesive layer may be applied as a single layer. Alternatively the adhesive layer can be applied as two or more multiple layers, which can have the same or different basis weights, thickness, fiber size, etc. In this aspect of the invention, advantageously two or more melt blown adhesive webs are used and include a layer of large diameter melt blown fibers (generally from about 20 to about 200 microns) applied initially to the carbon fiber structure, followed by a layer of smaller diameter melt blown fibers (from about 1 to about 75 microns). The resultant continuum of fiber sizes provides a variable porosity through the cross section of the web, thus allowing passage of ions for interaction with the carbon fibers, yet also preventing penetration of the web by carbon fibers. In addition, the larger diameter melt blown fibers can stick or adhere to the surface of the carbon fiber better than smaller diameter fibers. Still further, spraying the fiberized adhesive on opposing surfaces of the carbon fiber network can permit the formation of a naturally bonded edge, which in turn provides stability and protection without necessarily requiring a subsequent edge treatment.
The carbon fiber assemblies can further include one or more additional layers positioned so as to sandwich the adhesive layer between the additional layer and the carbon fiber network. Examples of additional layers include without limitation mesh scrims, nonwoven fabrics, such as spunbonded fabrics, and the like.
In one useful embodiment of the invention, the additional layer is a scrim material constructed using thermoplastic polymer coated yarns (preferably polyolefin coated fiberglass yarns, such as polypropylene coated glass yarns) and/or thermoplastic yarns. In this aspect of the invention, the scrim can be a woven or nonwoven substrate, such that the thermoplastic polymer coated yarns and/or other thermoplastic yarns are held into place mechanically. Alternatively, the scrim can be a nonwoven substrate in which the thermoplastic polymer coated yarns and/or other thermoplastic polymer yarns are adhesively bonded, for example by application of heat at the yarn contact or cross over points. Woven and nonwoven substrates in which yarns are held into place mechanically can include adhesive bonding at fiber cross over points as well.
The additional layer, such as a scrim material, can be bonded to the carbon fiber assembly via the adhesive layer of the carbon fiber assembly. For example, heat and optionally pressure can be applied to the carbon fiber assembly prior to or concurrently with directing the scrim into a face-to-face relationship with the adhesive layer. The use of woven or nonwoven scrims formed of thermoplastic polymer coated yarns and/or thermoplastic yarns provides the added benefit of still further improved bonding with the adhesive layer.
The additional layer(s) (such as a scrim) can also extend beyond one, preferably two, and more preferably three, edges of the assembly and the resultant assembly treated to form a selvage edge or seam as described above. For example, the selvage edge can be formed by applying heat and optionally pressure to the structure, which is particularly useful for those embodiments in which the additional layer(s) are scrims formed of polyolefin coated yarns and/or other thermoplastic yarns.
Advantageously, the carbon fiber network can include an adhesive layer and at least one additional layer, such as a mesh scrim as described above, on opposite sides thereof. For example, a representative carbon fiber assembly can include an open mesh scrim layer/adhesive layer/carbon fiber network/adhesive layer/open mesh scrim.
The carbon fiber assemblies of the invention can also include one or more edging materials, preferably formed of a low modulus polymer, positioned adjacent one or both longitudinal edge(s) of the carbon fiber network. The edging material can be used to control the width of the carbon fiber network, particularly for those embodiments employing carbon fiber tows. Also, such edging materials can provide additional dimensional stability to the assembly. Still further, the edging material can be selected so as to be compatible and/or have affinity with the adhesive layer so as to improve bonding of the adhesive layer to the carbon fiber assembly (for example, by selecting a polyolefin based adhesive as the adhesive layer and a polyolefin yarn or polyolefin sheathed or coated yarn as the edging material).
The present invention thus provides an insulating layer as an integral part of the carbon fiber network. In addition, an adhesive that is optionally fiberized and applied directly onto the carbon fiber network, particularly for a carbon fiber tow, can improve the cohesion and dimensional stability of the network. The adhesive layer can also provide significant adhesion with a carbon fiber substrate that includes a thermoplastic polymer coated fill yarn and/or other thermoplastic polymer fill yarn. The present invention not only provides an integral carbon fiber electrode/insulator material but also can provide a more stable, durable electrode structure. This in turn can improve ease of handling and fabrication into the desired end product, lower production costs and reduce production times.